1. Field of the Invention
This invention generally relates to solar cell fabrication and, more particularly, to a single heterojunction back contact solar cell and associated fabrication process.
2. Description of the Related Art
FIG. 1 is a partial cross-sectional view of a conventional silicon solar cell (prior art). The starting silicon wafer is usually lightly boron (p) doped. The emitter layer is formed by phosphorous (n) diffusion on the wafer front surface, and the back surface field is formed by either boron diffusion or by silicon-aluminum (Si—Al) eutectic formation. The front surface has antireflection coating (ARC) and surface texturing to reduce the light reflection and improve the cell efficiency. The front grid contacts the emitter layer (n-doped region) and the back metal contacts the back surface field (p-doped) region. The limitations of this cell are
(a) shading loss due to the metal grid;
(b) series resistance loss at the front metal grid and emitter resistance;
(c) front surface recombination loss at the emitter junction; and,
(d) rear surface recombination loss due to the low Al back surface passivation quality.
FIG. 2 is a partial cross-sectional view depicting a back contact solar cell (prior art). To reduce the front surface reflection loss and series resistance loss caused by the metal grid, the back contact solar cell was developed. For this cell, there is no shadowing effect, since there is no grid on the top surface. Series resistance is very low because the metal grid on the backside can be wide. Light trapping is improved because the front surface is decoupled from the electrical performance and only impacts the optical performance. Further, the cell has a simple electrical connection. A record cell efficiency of 23.4% has been reported. However, a high quality (single-crystal) silicon wafer is needed, so the photo-generated carriers can migrate to the wafer backside for collection. Single-crystal bulk silicon fabrication processes are expensive.
Although a high efficiency back contact cell has been reported, the open circuit voltage (VOC) for this cell is still less than 0.7V. This voltage clearly indicates that the surface recombination at the emitter junction (n+ to p) and at the base junction (p+ to p) are still high. To increase the VOC, it is necessary to reduce the surface recombination.
FIG. 3 is a perspective drawing depicting a heterojunction solar cell (prior art). It is generally agreed that heterojunctions create a minority carrier reflection mirror that can reduce the surface recombination and increase the VOC. For Si solar cell, the hydrogenated amorphous silicon to crystalline silicon (α-Si:H/c-Si) heterojunction of FIG. 3 has been extensively researched. α-Si:H has larger bandgap (1.7-1.9 eV) than Si (1.1 eV), and the heterojunction has a discontinuity at the conduction band (EC) and valance band (EV). A cell efficiency of 22.3% has been reported. Compared to the back contact cell, the heterojunction cell has larger VOC, but lower short circuit current (ISC) and lower fill factor (FF). The lower ISC is caused by the shadowing effect of the metal grid at the front surface, and the lower FF is due to the grid resistance.
A heterojunction is the interface that occurs between two layers or regions of dissimilar semiconductors. These semiconducting materials have unequal band gaps. The engineering of electronic energy bands is also used in the design of semiconductor lasers and transistors. When a heterojunction is used as the base-emitter junction of a bipolar junction transistor, extremely high forward gain and low reverse gain result. This translates into very good high frequency operation (values in tens to hundreds of GHz) and low leakage currents. This device is called a heterojunction bipolar transistor (HBT).
The principle difference between a bipolar junction transistor (BJT) and the HBT is the use of differing semiconductor materials for the emitter and base regions, creating a heterojunction at the interface. The effect is to limit the injection of minority carriers into the emitter region and increase emitter efficiency, since the potential barrier in the valence band is so large at the heterojunction interface. Unlike BJT technology, this allows high doping to be used in the base, creating higher electron mobility while maintaining gain.
It is a common assumption that a solar cell that combines the advantages of back contact cell and heterojunction cell would have a higher efficiency than either one by itself. While several papers discuss a double heterojunction back contact solar cell, no simple and inexpensive means have been reported for integrating n-type α-Si and p-type α-Si on a single side of a solar cell.
Additionally, an anomalous “S” shaped I-V characteristic is often observed for Si double heterojunction back contact cells fabricated both on p- and n-type c-Si wafers. This “S” shaped I-V characteristic has been attributed to various mechanisms including carrier recombination at the interface defects, recombination in the c-Si depletion region, offset in valence band, and offset in conduction band. The band offset may impose potential barriers for transport of photogenerated carriers across the heterojunction, thereby affecting the fill factor.
It would be advantageous if a solar cell could be efficiently fabricated that included the advantages of both a heterojunction and back contacts.